Implement VitsSVC resume training / finetune feature

[viewfinder-annn]

✨ Description:

This PR addresses bugs in VitsSVC module, mainly related to:

1. implement resume training / finetune feature
2. fix bug for multiple acoustic content training

🚧 Related Issues:

#91

👨‍💻 Changes Proposed:

• [x] Fix bug in models/svc/vits/vits.py, removed useless content_vec feature dependency.
• [x] Refractor VitsSVCTrainer to detach from TTSTrainer

🧑‍🤝‍🧑 Who can review?

@RMSnow @lmxue

🛠 TODO

• [x] Further clean up config
• [x] Refractor VitsSVCTrainer to detach from TTSTrainer

✅ Checklist:

• [ ] Code has been reviewed
• [x] Code complies with the project's code standards and best practices
• [x] Code has passed all tests
• [x] Code does not affect the normal use of existing features
• [x] Code has been commented properly
• [ ] Documentation has been updated (if applicable)

[viewfinder-annn]

Just fixed problem 1, now this PR doesn't affect tasks other than SVC.

[viewfinder-annn]

Now the default training data config is the same across DiffWaveNet and VitsSVC. The exp_config.json is now user-friendly with fewer exposed options. I'll refractor SVC preprocess to svc.json in another PR soon.